1. Field of the Invention
The invention relates to techniques for obtaining mineral fibers produced from a material having a high melting point such an basalt glass, glass based on blast furnace slag or any other similar glass composition. More precisely, the invention relates to the problem of conveying the material in the molten state from its melting chamber to a so-called free centrifuging fiber-drawing machine, i.e. a machine comprising one or more centrifuging wheels rotated at a high speed and onto the periphery of which there is poured the material to be drawn which is entrained by these wheels before some of the material is conveyed to the following wheel and the remainder is transformed into fibers under the effect of centrifugal force. The fibers are immediately entrained to a reception device by means of gaseous currents emitted perpendicular to the direction in which the fibers form.
2. Description of the Related Art
The techniques briefly mentioned above are used in order to produce so called rock wool which is used specially for manufacturing insulating products. They have the great advantage of enabling glasses with a high melting point to be processed, thus leading to products which are highly fireproof and with production and raw material costs which are relatively low, at least in comparison with those used in other fiber-drawing techniques. However, this moderate cost is to a large extent due to a certain simplicity of the technique, in particular as regards the conditioning of the glass.
In effect it is well known that fiber-drawing installations - in particular as regards the fiber yield and the quality of the fibers produced - are assessed more favorably, the more the fiber-drawing machine is supplied with a perfectly conditioned glass. What is understood by "conditioning" conforms to the reference value from the point of view of its composition, its temperature and its flow rate without any large irregularities being noted with respect to these features. Conventional glass-making ovens cannot ensure that this conditioning will be satisfactory since the refractories are rapidly corroded by the glasses used in this type of technique unless they accept temperatures of less than 1450.degree. C. at the furnace outlet while the optimum conditions for drawing fibers are at temperatures in the vicinity of 1600.degree. C. or even slightly more. In fact, in the fiber-drawing techniques examined here, the glass is usually melted in a chamber of the cupola type furnace in accordance with a method similar to that practiced in foundries, except for those differences due to the differing materials. The cupola furnace is a furnace which can produce particularly high flow rates for molten materials with a relatively cost-favorable fuel, e.g., coke, and very low maintenance costs. The cupola furnace is charged at the top alternately with layers of the material to be melted and layers of coke or directly with mixtures of coke and the material to be melted. The combustion of the coke, by means of air or pure oxygen blown into the lower part of the cupola, ensures that the heat required for the glass to melt is released. The molten mixture escapes continuously via a tapping hole located in the vicinity of this lower section. When operation has become established, the effect of a regular supply of alternating charges is in principle the formation of a stack of strata having a fixed thickness and level, the coke combustion and glass melting zone replacing this stack in the area of the nozzles.
Owing to the close proximity of the fusion zone and the nozzles which blow in the oxidizing agent, there is intense turbulence at the tapping hole which is due in particular to the permanent competition between the molten material and the combustion gases which are both attempting to escape. Owing to this fact, the flow is highly spasmodic. On the other hand, a cupola furnace is never as stable as one would wish it to be, and it can happen, for example, that a layer of coke flows violently instead of being gradually consumed with the consequence that blocks of unmelted coke are present in the molten material flow; apart from the great change in the flow rate of the molten material resulting therefrom, this discharge of coke is very harmful to the useful life of the centrifuging wheels which are generally made from refractory steel. In addition, instability of this type in the cupola furnace may lead to variations in temperature and, as a direct result, in the viscosity of the molten material and thus in its behavior during the fiber-drawing process.
In addition to these irregularities in the flow rate and temperature, there are irregularities in the composition of the molten material, if only from the point of view of the content of ferrous and ferric oxides, as the reduction of ferrous oxides to metallic iron is never complete and varies according to the more or less reducing nature of the atmosphere in the cupola furnace atmosphere; on the other hand, even if the melt produced by this reduction of the iron oxides is more dense than the molten glass and accumulates at the base of the cupola furnace, it is not rare to see it escape, although in small proportions it is true, with the flow of molten glass, giving rise to the considerable risk of damage to the centrifuging wheels.
In order to overcome these problems, it is known from French patent FR-B-2 572 390 to position in the path of the molten material a fore-hearth forming a reservoir having a free surface sufficient to at least partially attenuate variations in the flow rate at the outlet of this reservoir. The volume of this reservoir is preferably small and corresponds for example to an operating period of between 30 seconds and 3 minutes.
This small size is desirable in particular in order to restrict the cooling of the molten material held therein and the formation of dead areas, i.e. areas in which the material is in the devitrified state and no longer in the molten state, the fraction of the volume of the reservoir occupied by dead zones of this type being in fact unusable. Nevertheless, it is evident that the smaller the volume of the reservoir, the lesser the homogenization effect of the flow rate and possibly of the glass composition will be, and thus the more difficult it will be to control with any degree of accuracy the key parameters of the fiber-drawing process, i.e., the flow rate, temperature and composition of the molten glass.
In addition, even though the intensity of cooling is restricted, heat losses are no less real, the lowering of the temperature in the reservoir being of the order of 50.degree. to 100.degree. C. for example, which means that the material must be superheated to the same extent in the cupola furnace. Owing to its size and the very principle which controls its operation, the cupola furnace is a type of furnace which does not easily lend itself to extensive regulation of the temperature.
Moreover the fundamental difficulty with a reservoir of this type formed of a free surface with a flow via an overflow is that, by definition, it does not enable the flow rate to be controlled but only the variations thereof to be restricted. The average flow rate is thus imposed by the cupola furnace and cannot be modified with the degree of flexibility and speed desired in order to take account of the actual operating conditions of the fiber-drawing machine and in particular the position and distribution of the molten material stream on the first of the centrifuging wheels.
Patent application WO 90 02711 proposes operating such that material is melted in two stages, a melting stage in the cupola furnace and a superheating phase in the reservoir by means of a plasma heating device. The temperature of the molten material in the cupola furnace is therefore increased by the order of 20.degree. to 150.degree. C. relative to its outlet temperature. A system of this type enables the temperature of the molten glass conveyed over the first centrifuging wheel to be controlled efficiently but at the cost of a relatively complex heating installation which conflicts somewhat with the other, simpler components; the juxtaposition of components having very different technological values preferably being avoided on a production site. Moreover, the technique of heating by plasma can only operate efficiently on small amounts of molten glass, thus with a reservoir having a relatively small volume.
Apart from the disadvantages already cited above in this respect, it should be noted that in practice this also prevents the composition of the molten glass being altered in the reservoir. The possibility of adjustments of this type enables the cupola furnace to be supplied with a standard composition which in always the same while the final composition is adapted to the manufactured product or conversely any undesirable differences in the compositions of the raw materials to be corrected rapidly.
Furthermore, the fact should still be borne in mind that the basic problem of melting in a cupola furnace is that of the irregularity of the flow rates and this problem is only resolved to an unsatisfactory degree in the aforementioned PCT and French publications. In patent application WO-90 02711, the molten glass does not flow via an overflow but via a tapping hole located in the base of the reservoir protected by an immersed barrier which prevents the flow of the melt which, being more dense, is decanted at the base of the reservoir. The flow of molten material depends entirely on the height of the molten glass above the tapping hole, which height depends firstly on the flow rate of the supply to the reservoir and secondly on the dimensions of the reservoir or more exactly on the dimensions of its free surface area. The occupied section of a relatively small reservoir, of a size such that it can be heated using plasma, thus restricts the flow rate regulating capacities.
The same is true when the flow of glass is via a tapping hole at the base of the reservoir, with, moreover, additional difficulties connected with the wear of the tapping nozzles and thus the necessity to replace them, in particular owing to the highly corrosive nature of basalt glasses and the very high temperatures (far above 1000.degree. C.), and also connected with the great tendency of these glasses to devitrify at temperatures scarcely below the temperatures suitable for fiber-drawing, i.e. at which the viscosity of the glass is well adapted to a fiber-drawing process. Even if the nozzle is not cooled, there is a tendency for a crust of devitrified material to form on the nozzle on contact with the colder ambient air, this crust growing to a greater or lesser extent and reducing the passage cross-section of the glass; although this phenomenon may be overcome to a certain extent, it greatly complicates any attempt to regulate the flow rate by permanently altering the opening cross-section of the tapping aperture.
This devitrification phenomenon is particularly critical In the phases when the fiber-drawing machine is started up again since a plug forms which totally blocks the tapping aperture when the cupola furnace in tapped again. This plug must be removed by heating, for example by means of a blowpipe, until the flow is sufficiently great to provide the energy for maintaining the tapping process as a result of the developing crust being permanently leached. This heating stage using a blowpipe involves the risk of damaging the pouring nozzle as a result of overheating.
Furthermore, even in so-called continuous operation, the flow of molten material from the cupola furnace is regularly interrupted, in particular in order to tap off the molten iron accumulated at the base of the cupola furnace, which operation therefore interrupts the production of fibers at a moment which may be inconvenient. It is evident that this stoppage can only be avoided if the volume of the reservoir is relatively large and corresponds for example to a production period of the order of 5 to 10 minutes, the average time required for tapping off the molten iron.